Arrangement for collimating electromagnetic radiation

ABSTRACT

The invention relates to an arrangement for collimating electromagnetic radiation, comprising a macrocollimator which has at least two cutouts, and microcollimator structures which are positioned in the cutouts of the macrocollimator and have lamellae that absorb electromagnetic radiation, so that collimator channels are formed which in each case extend such that they are transparent in a transmission direction.

The invention relates to an arrangement for collimating electromagneticradiation, in particular X-ray radiation. The invention also relates toan X-ray detector and an X-ray device which are equipped with such anarrangement. Furthermore, the invention relates to a method of producingan arrangement for collimating electromagnetic radiation.

An arrangement for collimating X-ray radiation is known from patent U.S.Pat. No. 3,988,589. This arrangement consists of a number of individualelements which in each case consist essentially of a baseplate. Theplate sides have on one side grooves arranged at regular intervals andon the other side ridges (lamellae) arranged at regular intervals. Theindividual elements may be placed inside one another such that theridges of one baseplate engage in the grooves of a next baseplate,wherein channels are formed by the baseplates and the lamellae, saidchannels extending in a transmission direction. Such a collimator blockformed from a number of individual elements is placed in a frame in alast production step, wherein the frame has a cutout which extends inthe transmission direction and wherein the cutout is greater than thecollimator block. The free interspaces between frame and collimatorblock which extend in the transmission direction are then filled with aradiation-proof material (lead). Overall, a collimator with collimatorchannels for X-ray radiation which can be used in an Anger camera isthus provided.

It is an object of the invention to provide an arrangement forcollimating electromagnetic radiation which is suitable for largeradiation detectors.

This object is achieved by an arrangement for collimatingelectromagnetic radiation, comprising a macrocollimator which has atleast two cutouts, and microcollimator structures which are positionedin the cutouts of the macrocollimator and have lamellae that absorbelectromagnetic radiation, so that collimator channels are formed whichin each case extend such that they are transparent in a transmissiondirection.

Modern X-ray devices have increasingly large detectors. The dimensionsof a radiography detector may for instance be up to 50×50 cm², and thoseof a detector as used in computer tomography (CT) may be 100×4 cm². Evenmuch larger detectors of up to around 100×40 cm² are conceivable,particularly in the case of CT.

When examining relatively large objects by means of X-ray radiation,so-called scattered radiation is produced. Scattered radiation isproduced when X-ray quanta undergo an interaction with the object whichinteraction does not lead to absorption. Such interaction processes arefor example Compton scattering and Rayleigh scattering. In the case ofexaminations by means of X-ray, however, often only the unscatteredX-ray quanta are to be measured on the detector. Scattered X-ray quantagenerate a background signal which reduces the contrast and contributeto noise. In the case of large objects and large detectors, theproportion of scattered X-ray quanta may easily be 90% or more.

In other types of examination, the object itself is a source ofradiation, for instance in the case of single photon emission computedtomography (SPECT) or positron emission tomography (PET) or in the caseof dedicated measurements of scattered X-ray quanta. Each part of thedetector then receives X-ray quanta from each part of the object.However, meaningful measurements can often only be carried out when acertain detector part only receives radiation from an area of the objectdetermined by a collimation device.

In both problems, use is made of collimators which are arranged betweenthe detector and the object and serve to suppress certain parts of theX-ray radiation. Collimators have collimator channels which extend in alinear manner. A collimator channel consists of a radiation-transparentinner channel, or an inner channel that only absorbs radiation to aslight extent, and radiation-opaque collimator channel walls, orcollimator channel walls which absorb radiation to a greater extent.Each collimator channel is distinguished by extending in a transmissiondirection. The collimator channel walls border the inner channelessentially parallel to the transmission direction. The transmissiondirection may be the same for all collimator channels, for instance asin the case of a SPECT collimator in which all the collimator channelsare aligned parallel to one another, or else the transmission directionmay change from collimator channel to collimator channel, for instanceas in the case of a CT collimator, the individual collimator channels ofwhich are aligned on the focus point of an X-ray source. Radiation whichenters a collimator channel and differs in terms of its propagationdirection from the transmission direction of the collimator channel ishighly likely to be absorbed in the radiation-opaque collimator channelwalls. In local terms, a collimator therefore essentially allows throughonly radiation having a propagation direction which corresponds to thetransmission direction.

Collimators for collimating X-ray radiation are typically made from amaterial which greatly absorbs the X-ray radiation used, for instancefrom a heavy metal such as lead. Other metals may also be used, such astungsten, tantalum, molybdenum or alloys such as bronze with a high tincontent or compounds with a heavy metal such as tungsten oxide ortungsten carbide, or else use may be made of hybrid materials whichconsist for instance of a plastic matrix comprising embedded metalpowders. In the case of low-energy X-ray radiation (as used for examplein mammography), it is also possible to use copper, titanium or iron ormaterials with similar X-ray absorption.

In the case of CT or modern PET detectors, it is furthermore importantthat the individual grid channels are geometrically assigned preciselyto one detector element. The geometric precision of a large collimatorwith a large number of collimator channels can be maintained only withdifficulty and at a high cost. Cast or injection-molded components whichare cost-effective to produce have known precision problems atrelatively large dimensions, and these problems are manifested forinstance by shrinkage upon cooling and deformation with uneven cooling.Precise components which can be produced for instance by wire EDM oretching processes are extremely time-consuming and costly.

The collimator arrangement according to the invention has amacrocollimator which defines the overall geometry. Since themacrocollimator has cutouts for microcollimators, the macrocollimatorrequires only a small number of inner structures. The macrocollimatormay then be produced with high precision (for instance by wire EDM or bystacking etched metal sheets on top of one another) without entailinghigh costs. The fine structure of the collimator is produced by themicrocollimator structures. These may then be produced in inexpensivemethods (for instance by means of a casting process—e.g. lead casting orplastic injection molding, with it being possible for metal powder to beembedded in the plastic—or by simply placing sheets of metal inside oneanother in order to form a microcollimator with parallel collimatorchannels). The precision of the microcollimators must be sufficient onlyfor part of the overall collimator surface.

One embodiment of a collimator arrangement according to the inventionhas microcollimator structures which have collimator channels that atthe side (that is to say perpendicular to the transmission direction)are not completely enclosed by lamellae. The complete enclosure to forma collimator channel is achieved by the walls of the macrocollimatorwhen the microcollimator structure is positioned in the macrocollimator.In this way it is possible to make the macrocollimator walls as thick asthe lamellae thickness without the entire wall thickness between twoinner channels separated by a macrocollimator wall becoming greater thanthe wall thickness between two inner channels separated by a lamella ofa microcollimator structure.

In a further embodiment of a collimator arrangement according to theinvention, there is at least one guide structure. A guide structure aidsthe precise positioning of a microcollimator structure relative to themacrocollimator. A guide structure may be for example a groove or aguide rail.

In another embodiment of a collimator arrangement according to theinvention, there is at least one positioning structure. A positioningstructure is used for the precise positioning of the collimatorarrangement relative to an external unit, for instance a pixelateddetector. It is then possible to assign the collimator channelsparticularly precisely to the individual detector pixels, for examplesuch that the collimator channel walls are in each case positionedbetween two detector pixels and therefore a shading of the radiation onthe individual detector pixels by the collimator channel walls isavoided.

In one embodiment of a collimator arrangement according to theinvention, the cutouts are aligned in a focusing manner. In this way,microcollimator structures that collimate in a parallel manner and arecost-effective to produce can be positioned in the individual cutoutsand nevertheless an overall focusing of the collimator arrangement isachieved. Since collimator channels which are locally aligned inparallel lead to radiation shading in the case of focusing collimationthat is to be achieved overall, the geometry of the cutouts and of themicrocollimators must be selected such that an acceptable level ofshading is not exceeded.

A collimator arrangement according to the invention can beadvantageously used in an X-ray detector unit. In one embodiment of suchan X-ray detector unit, elements of the X-ray detector unit areconnected integrally with the microcollimator structures. In this way,an X-ray converter (e.g. a scintillator) may for instance in each casebe accommodated in a collimator channel.

The invention also relates to an E-ray device in which a collimatorarrangement according to the invention is used. This may be arranged inthe X-ray device for example in a manner such that it can be replaced oras part of the X-ray detector unit.

The invention furthermore relates to a method of producing a collimatorarrangement, wherein in one embodiment micro collimator structures areproduced by a casting or injection-molding process (for example a leadcasting process or a plastic injection-molding process).

The invention will be further described with reference to examples ofembodiments shown in the drawings to which, however, the invention isnot restricted

FIG. 1 shows a schematic diagram of a collimator arrangement accordingto the invention with macrocollimator and one microcollimator structureshown by way of example.

FIG. 2 shows an individual diagram of a microcollimator structure.

FIG. 3 shows a side view of a microcollimator structure with collimatorchannels aligned in parallel.

FIG. 4 shows an aspect of the microcollimator structure of FIG. 3.

FIG. 5 shows a side view of a microcollimator structure with collimatorchannels aligned in a focusing manner.

FIG. 6 shows an aspect of a macrocollimator with guide structures.

FIG. 7 shows an aspect of a macrocollimator, in the left cutout of whichthere is positioned one microcollimator structure and in the rightcutout of which there are positioned a number of microcollimatorstructures.

FIG. 8 shows a side view of a microcollimator structure with positioningstructures which allow positioning with respect to an external unit.

FIG. 9 shows a side view of a collimator arrangement with amacrocollimator aligned in a focusing manner and with microcollimatorstructures positioned in the cutouts, said microcollimator structureshaving collimator channels which are aligned in parallel.

FIG. 10 shows a side view of an X-ray detector comprising a collimatorarrangement according to the invention.

FIG. 11 shows an X-ray imaging device which is equipped with acollimator arrangement according to the invention.

FIG. 1 shows a schematic diagram of a macrocollimator 1 in which onemicrocollimator structure 2 is positioned by way of example in one ofthe cutouts 3.

FIG. 2 shows one embodiment of a microcollimator structure 2. Such amicrocollimator structure may be produced for instance in a casting orinjection-molding method. Lead casting and plastic injection-molding maybe mentioned here as examples. In a collimator for X-ray radiation, itis advantageous if in the plastic injection-molding method for exampleX-ray-absorbing powders (e.g. tungsten powder with particle sizes in themicrometer range) are incorporated in the plastic. Another method ofproducing a microcollimator structure is placing sheets that absorbelectromagnetic radiation inside one another. This can easily be done inthe case of collimator channels which are aligned in parallel. Themicrocollimator structure shown in FIG. 2 has transparent collimatorchannels which in each case extend in the transmission direction. Inthis context, transparent is to be understood as meaning that, forexample, even fixings with low radiation absorption (e.g. a fixing platemade of plastic which fixes the positioned microcollimator structures inthe macrocollimator) do not alter the transparency. In the embodimentshown, the radiation-transparent inner channels are formed by air andthe collimator channel walls are formed by lamellae, the extensiondirection of which is essentially the same as the transmission directionof the respective collimator channels.

The embodiment of a collimator arrangement according to the inventionshown in FIG. 1 shows that, given a suitably precise production of themacrocollimator, very large collimator arrangements can be produced withhigh overall precision and low costs. The costs for the precisemacrocollimator are low since the cutouts 3 may be selected to be largecompared to the desired collimator channels and therefore only a smallnumber of precise structures of the macrocollimator have to be produced.

FIG. 3 shows a side view of a microcollimator structure 2 withcollimator channels which are aligned in parallel (this is also referredto as a parallel collimator). The transmission direction runs in thedirection of the double arrow A. Parallel collimator arrangements areused for example to obtain a parallel projection image of an extendedsource distribution, for instance in the case of SPECT. The hatchedlamellae 4′ are in this embodiment to be understood as runningperpendicular to the plane of the paper. Lamellae 4″ (cf. FIG. 4) arearranged at regular intervals parallel to the plane of the paper, saidlamellae together with the lamellae running perpendicular to the planeof the paper bordering inner channels of collimator channels.

FIG. 4 shows an aspect of the microcollimator structure of FIG. 3. Thelamellae 4 enclose collimator channels 5 which are transparent in thetransmission direction, such that the collimator channels 5 have arectangular cross section. In the embodiment shown (which corresponds tothe side view in FIG. 3, with the side view being understood to be inthe direction of the arrow V), there are lamellae 4′ and lamellae 4″which run perpendicular to one another and as a result form therectangular cross section of the collimator channels 5. In theembodiment shown, at the sides of the microcollimator structure whichextend in the transmission direction there are formed collimatorchannels 5′ which are open at the side on account of not beingcompletely enclosed by lamellae. There may also be embodiments of amicrocollimator structure of the type shown which do not have anycollimator channels 5′ that are open at the side or which havecollimator channels 5′ that are open at the side on only one or two orthree sides.

FIG. 5 shows a side view of a microcollimator structure with collimatorchannels which are aligned on a point (this is also referred to as afocusing collimator). The hatched lamellae which run perpendicular tothe plane of the paper are aligned on a point. Such an embodiment of amicrocollimator structure is advantageous for example when the radiationfrom a point source, e.g. an X-ray source, is to be allowed through andradiation from other sources, for instance scattered radiation from anirradiated object, is to be absorbed in the lamellae. The lamellae whichrun in the plane of the paper either may extend parallel to the plane ofthe paper, which leads to focusing of the overall microcollimatorstructure on a line, or are likewise aligned on the source point, whichmeans that the lamellae are in each case arranged perpendicular to theplane of the paper at such an angle that all the collimator channels 5,5′ produced are aligned on a source point. The transmission directionfor each collimator channel then points in each case to this focuspoint.

Instead of the embodiments with rectangular collimator channels shownhere, collimator channels of different geometric shape may also beenclosed by the lamellae, for instance collimator channels of hexagonalor round cross section. The shape of the cross section of differentcollimator channels may also be different.

FIG. 6 shows an aspect of a macrocollimator I with two cutouts 3,wherein notches 6 are made at some points in the walls of themacrocollimator. FIG. 7 shows the collimator arrangement withmicrocollimator structures 2, 2′, 2″ positioned in the cutouts. Onemicrocollimator structure is positioned in the left-hand cutout, as isknown from FIGS. 3 to 5. The left-hand cutout is filled by a singlemicrocollimator structure. The notches 6 are used as guide structureswhich position the microcollimator structure relative to themacrocollimator. A precise positioning of the microcollimator structuresis facilitated by guide structures. Instead of notches, the guidestructures may also be formed by other structures known to the personskilled in the art, such as dents, or by guide rails which areadditionally attached. Furthermore, the walls of the macrocollimator inthis embodiment enclose the open collimator channels of the microgridstructure, so that completely enclosed collimator channels are formed.By means of open collimator channels, the situation is avoided wherebythe outer wall thickness of the microcollimator and the wall thicknessof the macrocollimator are added together. For a uniform size and auniform spacing of all collimator channels of the collimatorarrangement, the outer walls of the micro collimator structures wouldthen have to be made very thin compared to the thickness of thelamellae.

A microcollimator structure according to the invention may also havecollimator channels which are filled with a material that is onlyslightly absorbent, such as a polyurethane foam. This is advantageous inorder to increase the stability of the microcollimator structure. In oneembodiment, there are microcollimator structures which are produced froma block of a slightly absorbent material (for instance a hard foam)which has incisions into which absorbent lamellae are placed. In thisway, lamellae which are unstable per se (for example thin lead lamellae)may also be used, since the hard foam defines the stability. Even in thecase of filling with a slightly absorbent material, the collimatorchannels are to be regarded as transparent since the X-ray radiation isattenuated only a little within the slightly absorbent material comparedto the absorbent lamellae.

Various microcollimator structures are positioned in the right-handcutout of the macrocollimator in FIG. 7. In this embodiment which isshown by way of example, there are alternately comb sheets 2′ and flatsheets 2″ which in their entirety fill the cutout such that collimatorchannels are formed in this case too. In this embodiment, there aremicrocollimator structures 2″ which have neither closed nor opencollimator channels. Closed collimator channels 5 are formed only incollaboration with other microcollimator structures 2′ and the walls ofthe macrocollimator 1. Instead of comb sheets and flat sheets, sheets ofdifferent form may also be used as microcollimator structures if saidsheets can be placed in the cutouts such that collimator channels areformed. Such sheets may be for example deep-drawn sheets.

FIG. 8 shows a side view of a microcollimator structure 2 on whichpositioning structures 7 are fitted. The positioning structures 7 may inthis case have been formed integrally during the production process orbe attached subsequently. The positioning structures 7 allow thepositioning of the microcollimator structure 2 relative to an externalelement 10. In the embodiment shown, the positioning structures 7 engagein recessed parts of the external element 10. A precise alignment of thecollimator channels with respect to structures of the external element10 (for instance photodiodes for measuring electromagnetic radiation)can thus be achieved.

FIG. 9 shows a side view of a collimator arrangement with amacrocollimator 1 and microcollimator structures 2 positioned in thecutouts of the macrocollimator. In this embodiment, the macrocollimator1 is designed to be focusing, wherein the cutouts are designed such thattheir respective collimation directions are aligned on one point. If themicrocollimator structures, as in the example shown, are designed tocollimate in parallel, then the collimator arrangement nonetheless stillhas a focusing alignment overall on account of the macrocollimator.Depending on the height of the microcollimator structure, thecross-sectional area of the collimator channels and possibly othergeometric parameters, the size of the cutouts may be selected such thata focusing collimation of the overall collimator arrangement that isacceptable for the respective application is nevertheless produced. Theuse of parallel microcollimator structures offers the advantage that thelatter can be produced more easily than focusing microcollimatorstructures.

FIG. 10 schematically shows an X-ray detector in side view, in which acollimator arrangement according to the invention is used. Scintillatorphotodiode matrix modules 10 are arranged on a substrate 11. X-rayradiation which impinges on a scintillator and interacts with the latteris converted into optical light which the photodiodes measure andconvert into an electrical signal. The collimator arrangement isarranged between the detector and the radiation source.

FIG. 11 shows by way of example a medical X-ray imaging device 20 withan X-ray source 22 and an X-ray detector 21, in which a collimatorarrangement 23 according to the invention is used, said collimatorarrangement in this embodiment being arranged on the X-ray detector 21between the X-ray source 22 and the X-ray detector 21. The invention hasbeen described with reference to the preferred embodiments.Modifications and alterations may occur to others upon reading andunderstanding the preceding detailed description. It is intended thatthe invention be construed as including all such modifications andalterations insofar as they come within the scope of the appended claimsor the equivalents thereof.

1. An arrangement for collimating electromagnetic radiation, comprising:a macrocollimator which defines at least two cutouts, themacrocollimator defining a plurality of parallel notches on oppositefaces of each of the cutouts; and microcollimator structures which arepositioned in the cutouts of the macrocollimator and have lamellae thatabsorb electromagnetic radiation, so that collimator channels are formedwhich in each case extend such that they are transparent in atransmission direction, ends of at least some of the lamellae beingreceived in the macrocollimator notches.
 2. An arrangement as claimed inclaim 1, wherein the lamellae of the microcollimator structures define aplurality of closed collimator channels and along opposite sides defineopen collimator channels which perpendicular to the transmissiondirection are not completely enclosed by lamellae, lamellae of the opencollimator channels beign received in the macrocollimator notches andthe enclosure is completed by walls of the macrocollimator.
 3. Anarrangement as claimed in claim 1, wherein the cutouts are arranged in afocusing manner.
 4. An X-ray detector unit comprising an arrangement asclaimed in claim
 1. 5. An X-ray detector unit as claimed in claim 4,wherein at least one of the microcollimator structures is integrallyprovided with elements of the X-ray detector unit.
 6. An X-ray devicecomprising an arrangement as claimed in claim
 1. 7. A method ofproducing an arrangement for collimating electromagnetic radiation, saidmethod comprising the following steps: manufacturing a macrocollimatorwhich has at least two cutouts, manufacturing microcollimator structureswhich have lamellae that absorb electromagnetic radiation, inserting themicrocollimator structures in the cutouts so that collimator channelsare formed which in each case extend such that they are transparent in atransmission direction.
 8. A method as claimed in claim 7, wherein atleast one of the microcollimator structures has been produced in acasting or injection molding method.
 9. A method as claimed in claim 7wherein the macrocollimator is manufactured in a process separate fromthe microcollimators and subsequent to their manufacture themicrocollimators are frictionally received within cutouts defined by themacrocollimator.
 10. A method as claimed in claim 7 wherein themacrocollimator and microcollimators are manufactured separately.
 11. Acollimator having precise collimation channels for collimatingelectromagnetic radiation comprising: a macrocollimator encircling anddefining a plurality of cutouts which are large relative to thecollimator channels; a plurality of microcollimators having lamellaewhich define the collimator channels, the microcollimators eachconforming to a size of the cutouts and being configured to be insertedinto and received by one of the cutouts such that the macrocollimatorguiding the received microcollimators into an orientation in which thecollimator channels extend in an electromagnetic radiation transmissiondirection.
 12. The collimator as claimed in claim 11, wherein themicrocollimator defines positioning structures on a surface of each ofthe cutouts, the positioning structures interacting with themicrocollimator structures during insertion to position themicrocollimator structures relative to the macrocollimator.
 13. Thecollimator as claimed in claim 12, wherein the positioning structuresinclude guides extending along surfaces of the macrocollimator whichdefine the cutouts.
 14. The collimator according to claim 13, whereinthe guide structures includes notches or channels which extend parallelto the electromagnetic radiation transmission direction.
 15. Thecollimator as claimed in claim 11, wherein the lamellae are made of anelectromagnetic radiation absorbent material, and further including: amaterial which is only slightly electromagnetic radiation absorbentrelative to the material of the lamellae which fills the collimatorchannels.